Portable radiotelephones, such as pocket-sized cellular telephones and second generation (CT2) cordless telephones have become increasingly popular, especially as the size and the cost of such portable radiotelephones has been reduced. Current portable radiotelephones utilize an audible ringing signal to alert the portable radiotelephone user of an incoming call. There are, however, drawbacks to the use of an audible ringing signal. One such drawback is that when the portable radiotelephone is carried in a pocket, the audible ringing signal can become muffled, which can result in the portable radiotelephone user missing a call. Furthermore, there are many areas of public business, such as in theaters and in restaurants, where the use of devices providing an audible ringing signal are being banned, because the audible ringing signal is an annoyance to other customers at the establishment.
Tactile, or silent alerting devices have been utilized for some time in portable communication devices, such as pagers, to provide a vibratory alert signal. The tactile alerting device of choice in prior art portable communication devices has been a motor driven eccentric weight vibrator. While such motor driven eccentric weight vibrators have proved acceptable for use in many portable communication devices, they are generally unacceptable for use in current portable radiotelephones due to the increased space which is required to mount the motors. Also, most portable radiotelephones have only a very limited battery life, and the use of a motor driven eccentric weight vibrator which requires a significant current drain for operation would further reduce the operating time available for such portable radiotelephones.
Various linear resonant spring and non-linear hardening spring resonant armature systems have been described for use as tactile alerting devices, however each of these devices has limitations as to how much tactile energy can be reliably delivered. Because tactile alerting devices based on linear resonant springs are high-Q, the driving frequency must be tightly controlled to insure peak tactile energy output. And because tactile alerting devices based on non-linear hardening spring resonant armature systems have impulse outputs which varying with frequency, there is no repeatable single frequency which will reliably deliver the maximum tactile energy output.
Thus what is needed is an electronic driver for tactile alerting devices using linear resonant spring and non-linear hardening spring resonant armature systems.